The present invention relates to a method of operating a flue gas cleaning process and a flue gas filtering system utilizing filters susceptible to damage at temperatures exceeding a characteristic T.sub.max for the filters and including means for preventing such damage of the filters. The present invention also refers to a power generation method and a pressurized fluidized bed combustion system utilizing the flue gas filtering system.
The present invention more specifically relates to controlling the temperature of hot gases before leading them to a ceramic filter for particle removal. A need to control the temperature of hot gases exists particularly in combined cycle processes which utilize a pressurized fluidized bed combustor, and when there is a ceramic filter between the combustor and a gas turbine, which ceramic filter is easily damaged, when exposed to too high of temperatures.
In combined cycle processes, solid and liquid fuels can be utilized with high thermal efficiency. Thereby, fuel is combusted at, e.g., a 12-16 bar pressure in a fluidized bed combustor, whereafter the hot flue gases are exhausted from the combustor and allowed to expand in a gas turbine. In order to protect the gas turbine vanes from erosion and fouling, the flue gases are carefully cleaned before being introduced into the gas turbine. The cleaning is typically carried out in a gas purifier system including ceramic filters.
Ceramic filters are very proficient for separating fine solid particles, such as ash and sorbents, from the hot gases. It has, however, turned out that ceramic filters are highly sensitive to too high of gas temperatures at which the crystal structure of the material may change or its bonding agent may deteriorate, resulting in the filters becoming fragile and being broken. This has become a major problem. It is important that precautions are taken to avoid deterioration of ceramic filters, which are very expensive to repair, since the breaking of even a single filter tube in a combined cycle process can cause extensive damage to the gas turbine. Even small damage to the gas filter can be costly, as the whole plant may have to be stopped for removing a single damaged filter element, which easily may take several days.
The manufacturers of ceramic filters today usually state a highest recommended temperature for using the filters, which should--as a minimum precaution--be carefully followed. Typically, the combustion conditions are chosen so that a suitable flue gas temperature is obtained at the filter inlet. It would, however, in many cases be better if additional separate means were available for adjusting the filter inlet temperature. Given the fact that ceramic filter materials technology is still under development and the maximum allowable gas temperature at the inlet of a filter may in many cases be unknown, it would be desirable to have means of controlling the filter inlet temperature with a variable set point.
The thermal efficiency of combined cycle processes, such as pressurized circulating fluidized bed (PCFB) combined cycle processes, improves with increasing combustor flue gas temperatures, i.e., improves when the temperature of flue gases entering the gas turbine is increased. Therefore, it is not economical to cool the flue gases to a too low of a temperature level before introducing them into the ceramic filter. Thus, instead of cooling the flue gases considerably, it is recommendable to adjust the flue gas temperature such that the temperature remains at an as high of a temperature level as possible, but still below temperatures above which the life of the filter material will be significantly shortened or failure could result. These conflicting requirements, related to thermal efficiency and delicacy of the filter material, require that it is essential to maintain the inlet temperature below T.sub.max, but as close to T.sub.max as possible.
The possible fluctuations of the flue gas temperature, which may occur, e.g., due to variations of the fuel quality, make it necessary to leave a temperature margin, when adjusting the temperature of the flue gases. The temperature margin, between the aimed flue gas temperature and the highest recommended inlet temperature of the ceramic filter, has to be of the order of the amplitude of the largest expected temperature fluctuations of the flue gases. As such a temperature margin lowers the thermal efficiency, there is clearly a need for an as accurate and as rapid as possible flue gas temperature control means which can suppress the flue gas temperature fluctuations to 10.degree. F., preferably 5.degree. F. or even less. No such easily controllable adjustment means has until now been available.
British patent GB 2,261,831 discloses a method and an apparatus for protecting ceramic filter elements from sudden temperature variations by using heat storing elements to stabilize the temperature of flue gases before allowing them to come into contact with the ceramic filter elements. This method is not applicable for adjusting the temperature of the flue gases, only for damping its fluctuations.
European patent publication EP 0 611 590 suggests the introduction of an incombustible powder into flue gases prior to filtration, when a rise in flue gas temperature is foreseen. By this method undesired temperature rises may be suppressed, but as a side effect clogging of the filtration apparatus is accelerated.